LTC3554 LINER [Linear Technology], LTC3554 Datasheet - Page 19

• Manufacturer Part Number: LTC3554
• Description: Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators
• Manufacturer: LINER [Linear Technology]

OPERATION
Therefore, the hot trip point is set when:
and the cold trip point is set when:
Solving these equations for R
results in the following:
and
By setting R
in r
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
By using a bias resistor, R
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
where r
sired hot and cold trip points. Note that these equations
R
R
HOT
R
R
R
NTC|HOT
NTC|COLD
R
NOM
NOM
NOM
NOM
= 0.538 and r
HOT
R
R
NTC|HOT
=
+R
=
NTC|COLD
+R
and r
NOM
r
= 0.538 • R
0.538
r
3.17
COLD
NTC|HOT
= 3.17 • R
HOT
NTC|COLD
equal to R25, the above equations result
COLD
• R25
• R25
COLD
• V
are the resistance ratios at the de-
NOM
• V
NOM
BUS
= 3.17. Referencing these ratios
BUS
NOM
= 0.35 • V
= 0.76 • V
, different in value from
NTC|COLD
BUS
BUS
and R
NTC|HOT
are linked. Therefore, only one of the two trip points can
be independently set, the other is determined by the de-
fault ratios designed in the IC.
Consider an example where a 60°C hot trip point is desired.
From the Vishay curve 1 R-T characteristics, r
at 60°C. Using the above equation, R
to 46.4k. With this value of R
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in tem-
perature gain of the thermistor as absolute temperature
increases.
The upper and lower temperature trip points can be in-
dependently programmed by using an additional bias
resistor as shown in Figure 2. The following formulas can
be used to compute the values of R
For example, to set the trip points to 0°C and 45°C with
a Vishay curve 1 thermistor choose:
the nearest 1% value is 105k
The nearest 1% value is 12.7k. The fi nal solution is shown
in Figure 2 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
R
R
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
R
1
NOM
NOM
= 0.536 • R
=
=
r
3.266 – 0.4368
COLD
2.714
2.714
NOM
– r
HOT
– r
• R25
HOT
• 100k =104.2k
NOM
• R25
, the cold trip point is
NOM
NOM
LTC3554
and R1:
should be set
HOT
is 0.2488
19
3554f